\documentclass[a4paper]{article}

\usepackage{lipsum}
\usepackage{newtxtext}
\usepackage[17pt]{extsizes}

\title{\bfseries Changing the font size in  \LaTeX{}}
\author{Elegant\LaTeX{}}

\renewcommand{\ttdefault}{pcr}

\begin{document}

\maketitle

\begin{quote}
The sizes available are 8pt,9pt,10pt,11pt,12pt,14pt,17pt, and 20pt. There should be no need to change any other part of your document.
\end{quote}

\begin{verbatim}
\usepackage[20pt]{extsizes}
\end{verbatim}

\lipsum[3]

\end{document}